Corneal implants have a number of different applications. For example, corneal implants play various roles in refractive surgery. These roles include applications where the optical properties of the cornea are modified to achieve a desired effect, e.g. the correction of spherical error, astigmatic error, higher order aberrations or presbyopia. Corneal implants may also be used as prostheses or artificial corneas for replacement of corneal tissue. Throughout this application, the term “artificial cornea” shall mean a corneal implant that replaces at least some excised corneal tissue.
The preciousness of patients' vision and delicateness and intricateness of the human corneal structure and function mean that improvements in corneal implant technologies and implantation techniques are constantly sought after. For example, the inventor previously disclosed the advantages of reversibly deformable corneal implants which can be implanted through an incision size that is less than the maximum width of the implant. In addition, the inventor has previously disclosed reversibly deformable corneal implant prostheses that replace diseased corneal tissue and restore optical function. In some previously disclosed embodiments, these implants are reversibly deformable during implantation and sufficiently flexible to avoid damaging corneal tissue during the physiologic deformation of the cornea which occurs with blinking. Corneal implants and methods for their implantation into corneal pockets are also described in commonly owned U.S. Pat. Nos. 7,223,275 and 8,029,515; U.S. Patent Publication Nos. 2004/0243160; 2006/0173539; US2010/0069915; and PCT Publication No. WO 2008/055118. The disclosures of these applications are hereby incorporated by reference into this application in their entirety.
Most artificial corneas have been implanted in a penetrating fashion where the artificial cornea is in direct contact with the aqueous fluid in the eye. Because the synthetic materials which have been used for artificial corneas have not been able to completely integrate with the surrounding corneal tissue, bacteria from the surface of the eye can travel along microscopic openings between the artificial cornea and the circumscribing corneal tissue resulting in intraocular infection (endophthalmitis) which can cause loss of the eye.
In addition, extrusion of artificial corneas can result from any one or more of three primary causes herein. The first cause is the use of relatively large incisions for implantation. For example, the AlphaCor™ Artificial Cornea from Addition Technology, Inc., requires a 16 mm incision for implantation into an intralamellar pocket. The AlphaCor artificial cornea must be sutured in place with resorbable nylon sutures. The cornea has avascular tissue and heals poorly. Once the nylon sutures dissolve over a period of several years, the corneal scar tissue may not be sufficient in strength to hold the artificial cornea within the cornea, commonly resulting in extrusion.
A second potential cause of extrusion of artificial corneas is interference with corneal physiology. For example, some artificial corneas have been made from materials that are impermeable to both oxygen and glucose e.g. polymethylmethacrylate. Polymethymethacrylate has effectively no measurable oxygen permeability and therefore has a dK of zero Barrer. Over time the lack of adequate oxygenation and nutrition of the corneal tissues can result in melting of the cornea followed by extrusion.
A third potential cause of extrusion of artificial corneas is excessive rigidity of the artificial cornea. Very stiff materials, such as polymethylmethacrylate which has a Young's modulus between 1800 and 3100 MPa, can erode through the cornea over time. Such erosion can result from blinking of the eyelid which deforms the cornea and can abrade corneal tissue as the tissue repeatedly rubs against the rigid implant material. Such erosion can in turn lead to extrusion.
In addition to these problems, present artificial corneas can be uncomfortable for the patient. For example, the patient's tear film can be disrupted by the implant projecting above or falling below the surface of the surrounding cornea. Projection of the implant above the surface of the cornea can also cause abrasion of the inside of the eyelid. An implant with an optic below the surface of the surrounding cornea can also allow deposition of mucus into the “hole,” which can obscure the vision.
For these reasons, it would be desirable to provide improved artificial corneas which overcome at least some of the problems noted above. In particular, it would be desirable to provide artificial corneas and methods for their implantation where the risk of infection of the eye is reduced. It would be further desirable to provide artificial corneas which are resistant to extrusion due to any of the reasons cited above. Additionally, it would be desirable to provide artificial corneas which are comfortable for the patient and which maintain the tear film with minimum disruption. In addition, there is a need for artificial corneas made from a plurality of materials having disparate mechanical properties while being reversibly deformable (i.e. implantable through an incision that is smaller than the relaxed implant), and configured to replace excised corneal tissue. At least some of these objectives will be met by the disclosures described below.